Theoretically, a system in which a drug is converted to a metabolite that in turn is both rapidly excreted into urine and metabolized to a second metabolite in liver may be used to estimate hepatic blood flow rates. A search of the literature has suggested that an appropriate precursor-drug-metabolite combination for estimating functional hepatic blood flow might be aspirin-salicylic acid-salicyluric acid. We plan to study ( C-14)-labeled aspirin as the precursor and (H3)-labeled salicylic acid as the drug in rabbits, which normally excrete an alkaline urine. Alkaline urine is important because salicylate excretion in urine is much greater when the urine is alkaline than at acid pH.